Cathodic protection of steel in concrete has emerged in the last 10-15 years from being an experimental method to a well-established technique to combat reinforcement corrosion, particularly chloride-induced corrosion.
Various organizations have produced sets of criteria against which the performance of a cathodic protection (CP) system can be monitored and judged.
In some cases, the rigid adherence to specific criteria requested by some consulting engineers to judge the performance of CP systems has led to contractual conflicts and has contributed to setbacks in the growth and development of this technique.
The aim of this paper is to outline various practical issues related to the applicability of cathodic protection criteria under different circumstances.
Various empirical criteria have evolved over the years in the area of cathodic protection in reinforced concrete structures. Most of these criteria have been empirically determined by evaluating data obtained from successfully operated CP installations or have been developed based on laboratory experiments and in some cases they have developed for situations other than reinforced concrete. These criteria are now being globally accepted and form the basis of international and national standards.
This paper will present various practical issues related to the general applicability of these criteria under different circumstances.
PRACTICAL ISSUES
Application of Cathodic Protection
The decision to apply cathodic protection to a particular structure can be in many cases based on the results of a preliminary investigation that shows some high levels of chloride contamination in some elements of the structure. The structure receiving cathodic protection may have certain elements or even some regions of certain elements that in practice require CP as a preventative measure only. The steel in these elements is passive and the response of these elements to CP application can vary substantially depending on a combination of issues mainly related to concrete characteristics. The reduction in the corrosion rate that can be brought about by the CP application will be very minimal and achieving the 100mV decay criterion is not possible. In some cases, it is possible to achieve a particular criterion during commissioning. However, afterwards, due to combinations of conditions such as drying out of the concrete, initial low chloride level, low corrosion activity and passive steel, the voltage required to pass a given current will become high and only minimal or no current can be impressed. Under such circumstances, it is important to consider various factors such as the value of the decayed off potential and to consider the protection criteria applicability if such value is less negative than -150mV with respect to an Ag/AgC1 reference electrode.
The rigid adherence to CP criteria in this case is not appropriate. Careful consideration of the special conditions of the structure and engineering judgment should be applied to determine the appropriate assessment technique for the structure.
Current Density
The initial step for the design of a CP system is to determine the area of reinforcement to be protected. It is generally specified to base the design on a current density of 20mA/m2 of steel based on the area of the greatest steel density within the CP zone in order to ensure that anode design is adequate.
In some structures, it is impractical to fully adhere to the above requirements to select the design current density based on the greatest steel density due to the existence of small areas of high steel density in a particular CP zon
